Extracting hydrocarbons from subterranean formations requires a bore to be formed which extends from surface to intercept the formation. Such bores, when drilled, must be supported to prevent collapse, and sealed to prevent loss of fluid, such as drilling mud or hydrocarbons or the like, into the surrounding rock, or to prevent produced fluid from flowing to surface via an unintended flow path. This is conventionally achieved by providing lengths or “strings” of tubulars which are run into and cemented in place within the bore. Such bore-lining tubulars are generally referred to as casing or liner.
In conventional bore drilling operations, a bore is drilled to a depth of around, for example, 600 meters, when the drill bit and associated drill string is removed and a string of bore-lining tubing is run in. To secure and seal the tubing string within the bore a cement slurry is pumped down through the tubing string and back up into the annulus formed between the tubing and the bore wall. The cement then sets to secure and seal the bore. Drilling is recommenced for a further 600 meters, for example, following which a further tubing string is required to be cemented in place within the bore. This procedure is repeated until the bore reaches or nears the required total depth. Conventionally, each string of tubing extends back to, and is supported or hung from surface. Once the final drilling stage is completed the drilling string is pulled out of the hole and the final bore section is supported by a tubing, generally termed a liner, which does not extend back to the wellhead, but instead terminates downhole and is supported by the previous full string of tubing or casing. The support is provided by a liner hanger, as discussed in more detail below. The liner is also cemented within the bore.
Recent developments in the oil and gas exploration industry utilise expandable bore-lining tubing which enables “mono-bore” wells to be created. That is, tubing may be run into a newly drilled or “open” hole and positioned to overlap the lower end of existing bore-lining casing or liner. The newly positioned tubing is then radially expanded to an inner diameter substantially equal to that of the existing casing or liner, thus creating the so-called “mono-bore”. The existing casing or liner at its lower end supports each new tubing string.
As mentioned above, a liner hanger is utilised to secure a new tubing string to an existing tubing string within a bore. It is known in the art to establish such a liner hanger when utilising expandable tubing by radially expanding a portion of the new tubing into engagement with the lower end of the existing casing to create an interference coupling. However, in any such deformation of metallic tubing, there is a degree of elastic recovery which may prevent the desired degree of interference engagement being achieved, resulting in the creation of an ineffective liner hanger.
Due to the increasing utilisation of expandable casing and liner tubulars, various considerations must be observed to ensure that such expandable tubulars are properly cemented within the bore and that effective liner hangers, as required, are achieved. It is difficult to expand tubulars after a cementing operation, due to the expansion forces that would be required. Furthermore, expanding set cement will crack the cement, resulting in a loss of sealing function. If a casing string, for example, is required to be expanded after the cement slurry has been pumped into the annulus, care must be taken to ensure that the expansion operation is complete before the cement sets. It has been proposed, however, to utilise a cement which maintains a greater degree of compressibility than conventional cements once set. Furthermore, it is known to utilise apparatus which excludes cement from the area surrounding a portion of the tubular to be expanded. Such an apparatus is disclosed in Applicant's international patent application publication number WO02/25056, the disclosure of which is incorporated herein by reference. Otherwise, the bore-lining casing or liner must be cemented after expansion. However, cementing after expansion may also be difficult due to the reduced area of the annulus which may prevent the cement slurry from fully flowing around the exterior of the tubular, thus not properly sealing the tubular in the well bore. Additionally, the reduced annulus area may prevent or at least restrict the upward passage of fluid which generates the requirement for ports to be provided so that any fluid within the annulus may be displaced by the cement that is injected into the annulus.
It is among the objects of embodiments of the present invention to obviate or at least mitigate one or more of the above noted problems.